The present invention is directed generally to the transmission of signals in optical communications systems. More particularly, the invention relates to systems, devices, and methods for an efficient low power optical source driver.
The development of digital technology provides the ability to store and process vast amounts of information. While this development greatly increased information processing capabilities, it was soon recognized that in order to make effective use of information resources it was necessary to interconnect and allow communication between information resources. Efficient access to information resources requires the continued development of information transmission systems to facilitate the sharing of information between resources. One effort to achieve higher transmission capacities has focused on the development of optical transmission systems. Optical transmission systems can provide high capacity, low cost, low error rate transmission of information over long distances.
The transmission of information over optical systems is typically performed by imparting the information in some manner onto an optical carrier by varying characteristics of the optical carrier. In most optical transmission systems, the information is imparted by using an information data stream to either directly or externally modulate an optical carrier so that the information is imparted at the carrier frequency or on one or more sidebands, with the later technique sometimes called upconversion or sub-carrier modulation (“SCM”).
Initially, single wavelength carriers were spatially separated by placing each carrier on a different fiber to provide space division multiplexing (“SDM”) of the information in optical systems. As the demand for capacity grew, increasing numbers of information data streams were spaced in time, or time division multiplexed (“TDM”), on the single wavelength carrier in the SDM system as a means to better use the available bandwidth. The continued growth in demand has spawned the use of multiple wavelength carriers on a single fiber using wavelength division multiplexing (“WDM”). In WDM systems, further increases in transmission capacity can be achieved not only by increasing the transmission rate of the information on each wavelength, but also by increasing the number of wavelengths, or channel count, in the system.
There are two general options for increasing the channel count in WDM systems. The first option is to widen the transmission bandwidth to add more channels at current channel spacings. The second option is to decrease the spacing between the channels to provide a greater number of channels within a given transmission bandwidth. The first option currently provides only limited benefit, because most optical systems use erbium doped fiber amplifiers (“EDFAs”) to amplify the optical signal during transmission. EDFAs have a limited bandwidth of operation and suffer from non-linear amplifier characteristics within the bandwidth. Difficulties with the second option include controlling optical sources that are closely spaced to prevent interference from wavelength drift and nonlinear interactions between the signals.
WDM optical communication systems require a large number of optical sources. EDFA and Raman optical amplifiers, optical transmitters, and optical interface devices all require optical sources. The increasing demand for bandwidth requires an increased number of optical channels or optical wavelengths, which in turn increases the number of optical sources in the system. Additionally, the optical amplifiers require an increased number of optical sources to amplify the large number of optical channels effectively. Greater transmission distances also increase the number optical amplifiers required, and hence the number optical sources.
The optical sources typically used in optical communication systems exhibit variations in electrical characteristics from device to device. Also, the electrical characteristics of a device changes over time and operating conditions. In driving these various optical sources, the power supply provides power to accommodate the worst-case device characteristics. This leads to excess power dissipated as heat in resistive loads. While the excess power supplied to one optical source may not be significant, the excess power supplied to hundreds or thousands of optical sources becomes a problem. Generating this excess power increases system cost, weight, volume, and complexity. In addition, the excess heat must be dissipated increasing the need for additional heat sinks and cooling systems that increases system cost, weight, volume, and complexity.
Increasing the number of wavelength channels in WDM systems and the transmission bit rate have increased the power and cooling required to drive the greater number of optical source drivers. Therefore, a need exists for improved techniques and systems that provide more efficient, lower power optical source drivers in today's high capacity and long distance optical communication systems.